1. Field of the Invention
The invention is directed to metal oxide varistors (MOVs) having thermal protection, and more particularly, to MOVs which contain fusible materials which melt before the MOVs can begin thermal runaway.
2. Description of the Prior Art
In one known prior art device, thermal cut-off fuses are mounted electrically in series with the MOVs and adjacent one face of the MOVs. When the MOV heats up, due to the flow of current through the MOV, it causes a rise in temperature at such face which melts the thermal cut-off fuse which opens the electrical circuit to the MOVs. The thermal cut-off fuse being separated from the MOV surface can be erroneously heated by other nearby components, such as resistors, or erroneously cooled by convection currents in the surrounding housing.
In a further known prior art device, thermal cut-off fuses are located remote from the surface of the MOVs they are to protect and are connected to terminals which engage one face of the MOVs. Based upon the heating the terminals sense, their associated thermal cut-off fuse may be caused to operate. The terminals must have the desired response to heat and the factors of extraneous heating and cooling are also present.
Another device provides protection by utilizing varistors having a relatively low initial conduction voltage and using more of them, in parallel and in conductive relationship with a heat sink, for dissipation of the energy load imposed by multiple lightning strikes, for example.
Still another device uses current limiting fuses between the MOVs and ground. If the current through the fuse is sufficient, the fuse blows and actuates diagnostic circuitry.